Diehard Treo fans who pre-ordered their Treo 650s were dismayed to find that despite having the same amount of memory as the Treo 600, the Treo 650 cannot fit the same amount of data. A technical entry on the company's support site details the problem and suggests to plan on your files taking up 33% more on the Treo 650.
With NVFS, applications and data files may occupy a larger amount of memory on Treo 650 than they would on a handheld that uses volatile memory. With the Palm Operating System (Palm OS), each file contains internal records. The internal record size for files on NVFS is always exactly 512 bytes; records over 512 bytes are spanned across multiple records of 512 bytes. By contrast, with volatile memory, the record size can vary.
palmOne has admitted a problem and has offered a free 128 SD memory card to help ease the pain. You can read more on the issue over at Treo Central.